Partially hydrolyzed aluminum alkyl compounds known as aluminoxanes (AO) are used for activating transition metals for olefin polymerization activity. One such compound, methylaluminoxane (MAO), is a frequently chosen aluminum co-catalyst/activator in the industry. Considerable effort has been devoted to improving the effectiveness of catalyst systems based on use of aluminoxanes of modified aluminoxanes for polymerization of olefins. Representative patents and publications in the field of aluminoxane usage include the following: U.S. Pat. No. 5,324,800 to Welborn et al.; U.S. Pat. No. 4,752,597 to Turner; U.S. Pat. Nos. 4,980,878 and 5,041,584 to Crapo et al.; WO 96102580 to Dall'occo, et al.; EP 0 277 003 and EP 0 277 004 to Turner: Hlatky, Turner, and Eckman, J. Am. Chem. Soc., 1989, 111, 2728-2729; Hlatky and Upton, Macromolecules, 1986, 29, 8019-8020. U.S. Pat. No. 5,153,157 to Hlatky and Turner; U.S. Pat. No. 5,198,401 to Turner, Hlatky, and Eckman; Brintzinger, et al., Angew. Chem. Int. Ed. Engl., 1995, 34, 1143-1170; and the like. Despite technological advances, many aluminoxane-based polymerization catalyst activators still lack the activity and/or thermal stability needed for commercial applicability, require commercially unacceptably high aluminum loading, are expensive (especially MAO), and have other impediments to commercial implementation.
Many of the limiting features surrounding the use of aluminoxanes as activators for transition metals, for example, activity limitations—and the need for high aluminum loading, can be addressed by the use of stable or metastable hydroxyaluminoxanes. As compared to aluminoxanes, hydroxyaluminoxanes are generally highly active, provide reduced levels of ash, and result in improved clarity in polymers formed from such catalyst compositions. One representative hydroxyaluminoxane is hydroxyisobutylaluminoxane (HO-IBAO), which can be derived from the low-temperature hydrolysis of triisobutylaluminum (TIBA). Hydroxyaluminoxane compositions are disclosed in U.S. Pat. Nos. 6,562,991, 6,555,494, 6,492,292, 6,462,212, and 6,160,145.
In contrast to aluminoxanes, which appear to act as Lewis acids to activate transition metals, hydroxyaluminoxane species (generally abbreviated HO-AO) comprise active protons, and appear to activate transition metals by functioning as Brønsted acids. As used herein, an active proton is a proton capable of metal alkyl protonation. A typical hydroxyaluminoxane comprises a hydroxyl group bonded to at least one of its aluminum atoms. To form hydroxyaluminoxanes, typically a sufficient amount of water is reacted with an alkyl aluminum compound under appropriate conditions, for example at low temperature in hydrocarbon solvents, such that a compound having at least one HO—Al group is generated, which is capable of protonating a hydrocarbyl ligand from a d- or f-block organometallic compound to form a hydrocarbon. Therefore, polymerization catalysts derived from a hydroxyaluminoxane usually comprise: 1) a cation derived from a transition, lanthanide or actinide metal compound, for example a metallocene, by loss of a leaving group, and 2) an aluminoxate anion derived by transfer of a proton from a stable or metastable hydroxyaluminoxane to the leaving group. The leaving group is usually transformed into a neutral hydrocarbon thus rendering the catalyst-forming reaction irreversible.
One feature of hydroxyaluminoxanes is that their active protons are often thermally unstable when maintained in solution at ambient temperatures, likely due to the loss of active protons through alkane elimination. Thus, hydroxyaluminoxanes are frequently stored at temperatures lower than ambient temperature to maintain the active proton concentration. Typical low temperature storage is from about −20° C. to about 0° C. In the absence of such low temperature handling, the hydroxyaluminoxane activity decreases rapidly. Low-temperature storage is commercially cost prohibitive, especially over extended periods of time.
Thus, a need exists for hydroxyaluminoxane-type compositions that have more thermally-robust active protons, as compared to currently available hydroxyaluminoxanes, and that exhibit suitably high activity for commercial olefin polymerization.